If you keep telling yourself you couldn't possibly make someone else happy... then put it to the test.
Take a chance... flirt with the ladies... if one takes interest in you... follow where it may lead... if a relationship is formed... then put your best effort into it... and see if the person you're with is indeed happy.
It's a good bet... if you're wrong... then you've failed and actually made someone happy... wow.. imagine that! And if you're right, then you land right back in depression (how's that any different than now?).
You have nothing to lose and everything to gain. Prove yourself wrong... put your best effort into all of this... it's better than just moping around.. isn't it?
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen